This invention relates to a sensor device which uses a piezoelectric body vibrator to measure features of fluid in terms of coefficient of viscosity, specific gravity, density, etc.
The quality of fluid products, such as chemicals, foods, lubricating oil, car wax, etc., is controlled by their manufacturing process, and is guaranteed in terms of performance. It is important, therefore, to measure the features of such fluids in terms of the coefficient of viscosity, specific gravity, density, etc. In recent years, in order to measure the features of such fluids, it is proposed to make use of a piezoelectric body vibrator, as disclosed in U.S. Pat. No. 5,889,351. The disclosed device measures coefficient of viscosity of fluids and includes a piezoelectric body vibrator consisting of a piezoelectric body sandwiched by electrodes, a power source for applying a voltage for exciting vibration of the piezoelectric body vibrator, and an electric constant monitoring means for detecting changes in electric constants accompanied by vibration of the piezoelectric body.
In such a device, the piezoelectric body vibrator is made to vibrate in a fluid, which vibration is inhibited by mechanical resistance applied to the vibrator based on the viscosity of the fluid. Changes in electric constants of the piezoelectric body structuring the vibrator are detected, and the coefficient of viscosity of the fluid can be detected. Incidentally, the change in electric constants in the piezoelectric body is normally detected as a change in frequency of vibration in the piezoelectric body corresponding to a certain electric constant (e.g., phase) under predetermined conditions. Conventionally, for example, as shown in FIG. 6, a predetermined phase (xcex8) value (e.g., 70xc2x0) neighboring the resonance frequency (fmax) is adopted since it is difficult to detect the exact resonance frequency. The frequency fa where the phase value becomes xe2x80x9caxe2x80x9d (e.g., 70xc2x0) is obtained at one point at either side (left in the present example) of fmax. By precalibration, a vibrator vibrating in a fluid at frequency xe2x80x9cfaxe2x80x9d and phase xe2x80x9caxe2x80x9d can be said to have a corresponding viscosity.
Incidentally, even though fluids to be measured may be the same in terms of coefficient of viscosity and specific gravity, when the vibration characteristics of the piezoelectric vibrator change due to changes in the polarization state, additives to restrain vibration, changes in temperature, and the like, a change occurs in the shape of the frequency vs. phase curve. When the vibration characteristics change, in most cases, as shown in FIG. 7, the value of fmax remains unchanged, but the corresponding phase values change. For example, when xcex8 is xe2x80x9caxe2x80x9d, the frequency will dramatically change from fa to faxe2x80x2, resulting in erroneous measured values in the above-described technique.
In addition, since the piezoelectric body is initially polarized, that is, maintained at room temperature or higher temperatures under the Curie point for several hours to several days to stabilize the polarization state after an electric field with a level higher than a coercive field has been applied for a comparatively long period at a temperature close to the Curie point, the piezoelectric body to be used for a sensor device is extraordinarily highly sensitive. On the other hand, however the piezoelectric body maintains poor stability in many cases, and therefore, even if the polarization process described above is provided, a change in polarization state (deterioration in polarization state, depolarization) is apt to occur due to application of stress, passage of time, and the like. And, when such a change in polarization state occurs, as shown in FIG. 8, not only does the frequency vs. phase curve become flatter, as in FIG. 7, but it also shifts to higher frequencies. Accordingly, fmax shifts to fxe2x80x2max, thus resulting in greater error in measured values.
The purpose of the present invention, which has been made taking such conventional problems into consideration, is to provide a piezoelectric sensor device in which dispersion in measured values due to changes in vibration characteristics of the vibration system and the polarization state of the piezoelectric body can be reduced, as well as a method for detecting changes in electric constants of the sensor device.
According to the present invention, there is provided a piezoelectric sensor device comprising a piezoelectric body vibrator consisting of a piezoelectric body which is sandwiched by a pair of electrodes, a power source which applies a voltage to the piezoelectric body vibrator so as to vibrate the vibrator, means for monitoring electric constants to detect changes in electric constants accompanied by vibration of the piezoelectric body, the change in electric constants in the piezoelectric body being detected as a change in frequency for vibration of the piezoelectric body corresponding to an electric constant under predetermined conditions, and means for obtaining the resonant frequency, fmax, of the vibrator from frequencies at not less than two points at a predetermined electric-constant value.
Also according to the present invention, there is provided a method for detecting change in electric constants comprising: using the above-described piezoelectric sensor device, and obtaining fmax as an average value of two points, which give the same electric constant value.